Most women I know have an idea by monday midday of how they want to allocate their time and energy for the rest of the week. (this includes built-in time to get things done that need doing, and spend time alone to recharge if needed) but even if they can't shuffle around their plans, it's always a good sign when they counterrespond with "Well, I'm already committed for Friday, but I have next Tuesday open" (or what have you.)

Most of us just want to know that we aren't an afterthought (which is largely how we feel with last minute requests, unless its an opportunity that just popped up for the asker), and that someone is willing to put in a little effort!